This invention relates to an illumination system and a projection exposure apparatus. The present invention may be suitably used in an apparatus, called a stepper, for the manufacture of various devices such as IC""s, LSI""s, CCD""s, liquid crystal panels or magnetic heads, for example, wherein a circuit pattern formed on an original such as a photomask or reticle (hereinafter, xe2x80x9creticlexe2x80x9d) is illuminated uniformly with exposure light of ultraviolet rays or deep ultraviolet rays from an illumination system and the pattern is transferred by projection onto the surface of a wafer, which is coated with a photosensitive material.
In projection exposure apparatuses for the manufacture of semiconductor devices, a reticle having an electronic circuit pattern is illuminated with light from an illumination system, and the circuit pattern is projected and printed by a projection optical system onto the surface of a wafer. In order to attain a high resolving power, the illumination should be made to produce a uniform illuminance distribution within the range of illumination upon the reticle surface or the wafer surface.
For example, projection exposure apparatuses (steppers) may use an illumination system having a combination of a collimator lens and an optical integrator comprising small lenses arrayed at a predetermined pitch, by which exposure light is uniformly projected to the surface that is to be illuminated, such as a reticle surface or a wafer surface.
In an illumination system having such an optical integrator, a plurality of secondary light sources corresponding to the number of the small lenses is produced, and lights from these secondary light sources are superposedly projected, in different directions, to the surface to be illuminated, whereby a uniform illuminance distribution is produced.
Japanese Laid-Open Patent Applications, Laid-Open No. 193/1989, No. 295215/1989, No. 271718/1989, No. 48627/1990 and No. 270312/1998 show an illumination system having an internal reflection type integrator and an amplitude division type integrator (as described above), for enhanced uniformness of the illuminance distribution.
FIG. 9 is a schematic view of a portion of an illumination system having an internal reflection type integrator and an amplitude division type integrator, as disclosed in Japanese Laid-Open Patent Applications, Laid-Open No. 270312/1998.
In the drawing, laser light emitted from a laser light source 101 is once focused just before a light entrance surface of a light pipe 110 (internal reflection type integrator) by means of a lens system 107. After this, the light is diverged and it enters the light pipe 110 while defining a predetermined divergence angle with respect to the inside reflection surface of the light pipe.
The laser light entering the light pipe 110 is propagated while being reflected by the inside surface of the light pipe 110. Thus, the light pipe 110 produces a plurality of virtual images, of the laser light source 101, upon a plane (for example, plane 113) which is perpendicular to the optical axis.
At a light exit surface 110xe2x80x2 of the light pipe 110, plural laser beams coming from the virtual images (that is, laser beams which appear as apparently emitted from plural light sources) are superposed one upon another. As a result, a surface light source having uniform intensity distribution is produced at the light exit surface 110xe2x80x2 of the light pipe 110.
With an optical system including a condenser lens 105, an aperture stop 111 and a field lens 112, the light exit surface 110xe2x80x2 of the optical pipe 110 and a light entrance surface 106 of a fly""s eye lens 114 (amplitude division type integrator) are placed in an optically conjugate relation with each other. Thus, the surface light source of uniform intensity distribution produced at the light exit surface 110xe2x80x2 is imaged on the light entrance surface 106 of the fly""s eye lens 114, whereby light of uniform sectional intensity distribution is incident on the light entrance surface 106 of the fly""s eye lens 114. The fly""s eye lens 114 produces a plurality of light sources (secondary light sources) at its light exit surface. By means of a condenser lens (not shown) the lights from these light sources are superposed one upon another on a reticle (not shown), by which the pattern of the reticle as a whole is illuminated with uniform light intensity. In FIG. 9, denoted at LF is the imaging light flux of the optical system (105, 111, 112) and denoted at NA is the numerical aperture of the optical system at the light exit side.
The shape of the light pipe 110 is determined by taking into account the divergence angle of the laser light from the lens system 107 as well as the length and width of the light pipe 110, so that the laser beams projected from the light sources to various points on the light entrance surface 106 have optical path differences greater than the coherence length of the laser light. This reduces the coherence with respect to time, thereby suppressing speckle (interference fringe) produced on the light entrance surface 106.
The manufacture of recent large-integration semiconductor devices such as VLSI requires extraordinarily high uniformness of illuminance distribution for the circuit pattern printing. On the other hand, the whole optical system is required to provide an increased transmission factor in order to reduce a loss in the quantity of exposure light. However, in the illumination system shown in FIG. 9, in order to produce a uniform surface light source at the light exit surface of the light pipe, the number of times of inside reflection of the divergent light should be large. To this end, the length of the light pipe should be enlarged while holding the diameter fixed. However, this elongation causes a decrease of the transmission factor due to absorption. Therefore, the length cannot be enlarged, beyond a certain limitation.
This means that improving the uniformness of the illuminance distribution may cause a decrease in the transmission factor, and that retaining a good transmission factor may demand shortening the length of the light pipe, which leads to a failure of the uniform surface light source.
It is an object of the present invention to provide an illumination system by which a light pattern having a uniform sectional light intensity distribution can be produced without considerably decreasing the transmission factor of an optical system.
It is another object of the present invention to provide a projection exposure apparatus having an illumination system such as described above.
In accordance with an aspect of the present invention, there is provided an illumination system, comprising: a light source; an emission angle preserving optical element for emitting light from said light source, at a certain emission angle; a collecting optical system for collecting the light from said emission angle preserving optical element; a pattern forming optical system having at least one of a relay optical system and a diffractive optical element, for producing, on a predetermined plane, a light pattern of desired shape having a uniform light intensity distribution, by use of light from said light collecting optical system; multiple-beam producing means; a zooming optical system for projecting the light intensity distribution on the predetermined plane, onto a light entrance surface of said multiple-beam producing means at a predetermined magnification; and illuminating means for superposedly projecting lights from a light exit surface of said multiple-beam producing means, upon a surface to be illuminated.
In one preferred form of this aspect of the present invention, another emission angle preserving optical element having a different divergence angle is used interchangeably in response to a change in projection magnification when the light from the predetermined plane is projected by said zooming optical system to the light entrance surface of said multiple-beam producing means, to thereby adjust a numerical aperture of light entering said multiple-beam producing means.
There may be plural diffractive optical elements each being as aforesaid, and one of the diffractive optical elements may be selectively disposed on a light path to change the illuminance distribution on the light entrance surface of said multiple-beam producing means.
The emission angle preserving optical element may include a fly""s eye lens having small lenses disposed two-dimensionally.
The diffractive optical element may comprise a computer generated hologram of phase type or amplitude type.
The multiple-beam producing means may include a fly""s eye lens having small lenses disposed two-dimensionally, by which light incident thereon is divided into a number of light fluxes when emitted therefrom.
In accordance with another aspect of the present invention, there is provided an illumination system, comprising: a first optical system for forming, with light from a light source, a plurality of secondary light sources; and a second optical system for superposedly projecting lights from said secondary light sources onto a surface to be illuminated; wherein said first optical system includes a diffractive optical element, such as computer generated hologram, for example, for supplying a light pattern having a uniform light intensity distribution and having a shape corresponding to said secondary light sources.
In accordance with a further aspect of the present invention, there is provided a projection exposure apparatus, wherein a reticle having a pattern is illuminated with light from an illumination system as recited above, and wherein the pattern of the reticle as illuminated is projected on a substrate to be exposed.
In accordance with a yet further aspect of the present invention, there is provided a device manufacturing method, comprising the steps of: illuminating a device pattern of a reticle with light from an illumination system as recited above; exposing a wafer with the device pattern as illuminated; and performing a development process to the exposed wafer.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.